**RMC modelling**

The model consists in 1950 atoms in a cubic box (30.12 Å length for corresponding to the number density

F(Q) = [I_{coh}(Q)-<f^{2}>] / <f>^{2},

where the <f^{2}> and <f>^{2} are the usual
mean diffusion factors, depending on Q (X-ray) or not (neutron). About
16 million accepted moves were completed before the sixfold constraint
was fulfilled, the last of the 300 octahedra was particularly long to be
obtained. The whole process took 720 hours CPU on a DEC-ALPHA AXP 4620.

The R factors calculated as 100*|Iobs-*k*Icalc|/|Iobs| (%) (according
to the definition I(Q) = F(Q)+1, *k* being a scale factor) are 1.42,
1.29 and 1.72 % respectively for the Fe and V-based neutron data and the
Fe-based X-ray data. The observed and calculated F(Q) curves are shown
on the figure 1. A global view of the octahedra network
is shown on the figure 2.

**Fig. 1.** Experimental (dots) and RMC
simulated (solid line) TSF of NaPbM_{2}F_{9}; (a) neutron
data for M = Fe, (b) neutron data for M = V, (c) X-ray data for M ) Fe.

**Fig. 2.** Projection of the RMC cubic
box showing the 300 [MF_{6}] polyhedra network.

**-/0/+
Armel Le Bail
- June 1997**